1. Field of the Invention
This invention relates to a method of preparing a laminated packaging material. More particularly it relates to a method of preparing a laminated packaging material comprising a thin-film layer of an inorganic or metal compound.
2. Description of the Related Art
Packages for food or medical supplies have been hitherto commonly prepared in the form of a pouch, which is a baglike container receptacle. As materials therefor, those mainly comprised of a plastic film are in wide use.
Such packaging materials are required to have various functions and performances according to their contents. In particular, they are often required to have gas barrier properties so that the contents can be prevented from deterioration due to oxidation, the active components in the contents as exemplified by perfume components or pharmaceutical active ingredients can be prevented from permeating or escaping through the package or the contents can be prevented from drying. In order to impart the gas barrier properties to packaging materials, a plastic film that forms a substrate of the packaging material is usually laminated with a layer comprised of a material with good gas barrier properties. A thin-film layer of polyvinyl alcohol, polyvinylidene chloride, metal foil, a metal or an inorganic or metal compound has been used as the material with good gas barrier properties. In particular, a thin-film layer of silicon oxide has an excellent transparency, shows very high gas barrier properties, and yet can be produced at a relatively low cost. Hence, its practical utilization has been researched in variety (Japanese Patent Publications No. 51-48511 and No. 53-12953, Japanese Utility Model Publications No. 52-3418 and No. 52-24608, etc.).
The transparent thin-film layer of an inorganic or metal compound as disclosed in the above Japanese Patent Publication No. 51-48511, Japanese Utility Model Publication No. 52-24608, etc., however, is comprised of so thin a glassy layer that fine cracks tend to occur, which seriously tend to occur especially when heated. This is presumed to result from the phenomenon that the substrate on which the transparent thin-film layer is formed undergoes expansion and contraction by heat and the transparent thin-film layer can not follow this expansion and contraction. Occurrence of such cracks results in a serious lowering of the gas barrier properties of the packaging material.
An occasion on which the thin-film layer of an inorganic or metal compound is heated can be met when a thermoadhesive resin layer is formed on the thin-film layer in order for the packaging material to be endowed with thermal adhesion properties. As a function required in packaging materials, packaging materials must have the thermal adhesion properties so that the packaging materials can be readily worked when they are formed into baglike packages. In order for packaging materials to be endowed with the thermal adhesion properties, the substrate for a package is laminated with a thermoadhesive resin film (also called a sealing medium) in usual instances. In such instances, the sealing medium usually gives a layer having the largest thickness among the layers that constitute a package, and also gives a layer that imparts strength and flexibility to the packaging material and serves as a main component layer of the package. A method for its lamination may include a method in which the substrate is laminated with the thermoadhesive resin film (the sealing medium) previously prepared, and a method in which a thermoadhesive resin is directly extruded in the form of a film onto the substrate to effect lamination to form the sealing medium, i.e., what is called the extrusion coating. In the method in which the substrate is laminated with the sealing medium, a substrate on which the thin-film layer of an inorganic or metal compound has been formed and the thermoadhesive resin film (sealing medium) previously prepared may be laminated using an adhesive, or the substrate and the sealing medium are laminated interposing between them a thermoadhesive resin coating formed by extrusion. The latter enables easy formation of the sealing medium in a large thickness, and is a preferred method. This is because, in the method in which the substrate and the sealing medium are laminated interposing an adhesive, the presence of the adhesive may cause an increase in rigidity of sheets, a decrease in flexibility thereof, and a lowering of transparency.
Almost all of the packages making use of metal foil which are commercially available at present are comprised of a sealing medium formed by extrusion coating of the thermoadhesive resin onto a metal foil.
However, as previously stated, in the direct extrusion coating of the thermoadhesive resin onto the thin-film layer, the substrate may be expanded because of the mechanical tension applied when coating is carried out and the heat of molten thermoadhesive resin, to cause occurrence of fine cracks in the thin-film layer. In addition, the occurrence of cracks may be promoted because the thermoadhesive resin layer formed by extrusion coating and the substrate heated undergo constriction as they are cooled. As a result, the gas barrier properties of the package may be greatly lowered. This has been a problem unsolved.